Select embodiments of the present invention may be used to measure stress in tensioned members of critical structures. This measure of stress is also referred to as “bulk tension.” In many cases access to these members is limited, e.g., steel reinforcing members buried in concrete. Critical structures include dams, bridges, elevated highways, nuclear containment domes, parking garages, piers, tunnels, and the like.
Acoustic waves are nondestructive and are capable of traveling long distances in engineered structures. Further they can be used to “interrogate” a structure to determine its integrity. Acoustic “interrogation” signals may be employed for purposes of determining bulk properties and to detect defects. Bulk properties, such as tension, are determined by acoustic signals interacting macroscopically with material, whereas, defects are identified by acoustic signals interacting microscopically with material. These dual purposes are achieved by carefully shaping transmitted acoustic signals and using tailored signal processing techniques on the reflected signals. Acoustic interrogation can identify both bulk properties and defects, quantifying results quickly, i.e., in “near real-time” although custom processing may extend display of results by one or two minutes.
There are two common types of ultrasonic waves, longitudinal and shear. Other types of ultrasonic waves exist, such as surface waves and plate waves. For a longitudinal wave, also termed compressional wave, particles vibrate in a direction that is the same as the propagation direction. For a shear wave, particles vibrate in a direction that is perpendicular to the propagation direction. Shear wave velocities, Vs, are typically about half of longitudinal wave velocities, Vl. Shear waves do not exist in some media, such as water and air, although solid media support shear waves.
Landa and Plesek employed shear waves in a technique that is both reasonably sensitive and linear. Landa, M. and J. Plesek, Ultrasonic Techniques for Non-Destructive Evaluation of Internal Stresses, Institute of Thermomechanics ASCR, Dolejskova 5, 18200, Praha8, Czech Republic, October, 2000. Their technique is limited to using shear waves that are polarized in two directions, parallel to the principal stress axis and transverse to the principal stress axis. These shear waves propagate in the remaining direction across the principle stress axis. Propagation parallel to the principal stress axis is preferable.
A select embodiment of the present invention now enables inspectors to quickly and easily make a quantitative determination of damage or degradation of post-tensioned members or objects. Prior to the present invention, two methods were commonly available for this purpose. The first is a “hammer test” that produces a first acoustic tone when the object is under zero or low tension and a second noticeably different tone when under designed (moderate or high) tension. Obviously, the hammer test yields a purely qualitative result. The second method involves the use of a jack, such as a hydraulic jack and is termed a “jacking test.” It often requires attaining “reasonable” access to members that otherwise have limited access. Jacking is both laborious and expensive when used to determine the condition of post-tensioned members in the field. While the jacking test is quantitative, it cannot be used in many situations because of restricted access considerations, expense, or both.
U.S. Pat. No. 5,154,081, Means and Method for Ultrasonic Measurement of Material Properties, to Thompson et al., Oct. 13, 1992 employs two electromagnetic acoustic sensors arranged on a single side of an object to be measured. Stress measurements are limited to those available near a surface of ferromagnetic objects having a large accessible surface. No measurements are made throughout the bulk of the object.
U.S. Pat. No. 5,289,387, Method for Measuring Stress, to Higo, et al., Feb. 22, 1994, uses a variety of sensor types and placements on metal, polycarbonates or acryl resin objects to measure bulk stress. The '387 patent measures attenuation of ultrasound to determine stress. Since attenuation is an indirect measurement, i.e., not related to fundamental ultrasonic properties, this method is limited to measuring very well characterized objects, such as standard items in a production line. For example, it cannot be used successfully on unknown parts picked at random.
U.S. Pat. No. 6,477,473, Ultrasonic Stress Measurement Using the Critically Refracted Longitudinal (LCR) Ultrasonic Technique, to Bray, Nov. 5, 2002, uses two sensors in a specific arrangement placed on a single side of an object. The sensors measure a reflection angle to determine longitudinal wave speed and hence stress. This device is limited to objects with accessible large surfaces since it measures the velocity of a longitudinal wave only.
None of these patents provide a device or method for determining tension in a randomly picked object that may have only a limited surface available for access, such as a reinforcing member embedded in concrete. Embodiments of the present invention differ from existing ultrasonic instruments, such as the StressTel®, BoltMike® and the like, that measure tension in bolts. These instruments measure elongation of a bolt while it is being torqued. They are “tension (bolting) control systems” that depend upon measuring changes in length between the un-loaded and the loaded (stressed) conditions of a particular fastening device, such as a bolt. Thus, unlike an embodiment of the present invention, they cannot measure stress in a fastener, such as a bolt or screw, that was tightened prior to use of the instrument.
From first principles of ultrasonic theory a relationship for calculating stress (tension) in a part using only the shear and longitudinal velocities may be derived as follows:
                    σ        =                              (                                          V                l                2                            -                              2                ⁢                                  V                  s                  2                                                      )                                2            ⁢                          (                                                V                  l                  2                                -                                  V                  s                  2                                            )                                                          (        1        )            where Vl is the longitudinal wave velocity and Vs is the shear wave velocity and σ is the bulk stress (tension) along the principal stress axis of the structure to be measured. However, Eqn. (1) has been relegated to theory and not adapted for use because heretofore both shear and longitudinal velocities were unable to be measured simultaneously and accurately. Select embodiments of the present invention address this limitation by employing Eqn. (1) in the design of a robust, portable, efficient and relatively inexpensive package. Further, embodiments of the present invention do not require using shear waves that are polarized in two directions, i.e., parallel to the principal stress axis and transverse to the principal stress axis, to propagate in the remaining direction across the principle stress axis. Instead, applying Eqn. (1) allows calculation of bulk stress (bulk tension) by using acoustic energy, preferably ultrasound, propagated parallel to the principal stress axis.
Select embodiments of the present invention are able to address a wider range of situations than is possible using prior techniques. An embodiment of the present invention is useful for accurately determining the bulk stress inside an object that may offer limited access in its permanent installation, such as a reinforcing member buried in concrete, a post-tensioned element used in dams or bridges, and the like. An accurate measure of the bulk stress in a reinforcing member is critical for determining the structural integrity of damaged buildings; in making “repair or replace” decisions on existing structures; in determining the extent of deterioration of infrastructure; in researching degradation of materials, and in like applications. An embodiment of the present invention is also useful for accurately determining the strength of undamaged walls, e.g., resistance to penetration.
Select embodiments of the present invention may be provided in a portable package. Further, select embodiments of the present invention are able to limit the imposition of the acoustic signal to small areas, essentially points, for improved resolution.